1. Field of the Invention
This invention relates to improvements in the production of reactive electron-acceptor pigments useful in so-called "carbonless copying paper" systems in particular to improvements in the production of the reduced charge montmorillonite pigments useful as color formers in record material as disclosed and claimed U.S. Pat. No. 3,993,500. These pigments are obtained by replacing exchangeable cations of clays, a substantial proportion of which is dioctahedral montmorillonite, with a controlled amount of lithium ions, mildly heating the ion-exchanged montmorillonite to collapse irreversibly the montmorillonite structure and grinding to pigment-size particles. The teachings of said application are incorporated herein by reference thereto.
2. Prior Art
"Reduced charge montmorillonites," as described in the literature, embraces a series of montmorillonites of progressively lower charge, as determined by lattice expansion characteristics and quantity of exchangeable monobasic cations. Members of the series are obtained by exchanging exchangeable cations of dioctahedral montmorillonite with various amounts of lithium cations, followed by mild heat treatment to collapse the mineral lattice and fix the exchanged lithium cations (render lithium ions nonexchangeable). A rationale for the development of the unique properties of reduced-charge montmorillonites has been provided in the literature, specifically by G. W. Brindley and G. Ertem, CLAYS & CLAY MINERALS, 1971, Vol. 19, pages 300 to 404; G. Ertem, ibid, 1972, Vol. 20, pages 199 to 205; and R. Calvet and R. Prost, ibid, 1971, Vol. 19, pages 175 to 186.
Reduced charge montmorillonite has been produced by ion-exchanging cations in suitable montmorillonite clay with lithium ions derived from lithium chloride, washing the ion-exchanged clay to remove salts and heating the washed material. Use of lithium chloride or lithium salts of acids conventionally used to ion-exchange clays and the like is not practical in commercial production of a reduced charge montmorillonite pigment because of the difficulties involved in filtering the lithium-exchanged intermediate and washing the filtered solids to remove excess lithium salt. If not removed, the excess lithium salt would interfere with the pigmented coating composition. Thus, bentonite with calcium as the major exchangeable cation can be ion-exchanged with lithium chloride at clay solids of 30% or higher in some cases but the exchanged clay filters slowly using conventional filters such as vacuum filters. As soon as most of the salt has been removed from the filtered ion-exchanged clay by washing with water, it is extremely difficult to filter the wash water due to the removal of the strong electrolyte which permitted filtration, albeit slow, of the slurry of ion-exchanged clay. This technique for producing reduced charge montmorillonite pigment on a large commercial scale is obviously impractical. Aside from the expense involved or waste of lithium salts during ion-exchange, the low filtration rates would necessitate large capital investment in filtration equipment.
Accordingly, an objective of the invention is to provide a simple process for producing a reduced charge montmorillonite pigment whereby losses of the source of lithium ions are eliminated and filtration and washing are greatly facilitated.